Ligaments are tough bands of tissue which serve to connect the articular extremities of bones, and/or to support and/or retain organs in place within the body. Ligaments are typically made up of coarse bundles of dense fibrous tissue which are disposed in a parallel or closely interlaced manner, with the fibrous tissue being pliant and flexible but not significantly extensible.
In many cases, ligaments are torn or ruptured as the result of an accident. Accordingly, various procedures have been developed to repair or replace such damaged ligaments.
For example, in the human knee, the anterior and posterior cruciate ligaments (i.e., the “ACL” and “PCL”) extend between the top end of the tibia and the bottom end of the femur. The ACL and PCL serve, together with other ligaments and soft tissue, to provide both static and dynamic stability to the knee. Often, the anterior cruciate ligament (i.e., the ACL) is ruptured or torn as the result of, for example, a sports-related injury. Consequently, various surgical procedures have been developed for reconstructing the ACL so as to restore substantially normal function to the knee.
In many instances, the ACL may be reconstructed by replacing the ruptured ACL with a graft ligament. More particularly, in such a procedure, bone tunnels are generally formed in both the top of the tibia and the bottom of the femur, with one end of the graft ligament being positioned in the femoral tunnel and the other end of the graft ligament being positioned in the tibial tunnel, and with the intermediate portion of the graft ligament spanning the distance between the bottom of the femur and the top of the tibia. The two ends of the graft ligament are anchored in their respective bone tunnels in various ways well known in the art so that the graft ligament extends between the bottom end of the femur and the top end of the tibia in substantially the same way, and with substantially the same function, as the original ACL. This graft ligament then cooperates with the surrounding anatomical structures so as to restore substantially normal function to the knee.
In some circumstances, the graft ligament may be a ligament or tendon which is harvested from elsewhere within the patient's body, e.g., a patella tendon with or without bone blocks attached, a semitendinosus tendon and/or a gracilis tendon. In other circumstances, the graft ligament may be harvested from a cadaver. In still other circumstances, the graft ligament may be a synthetic device. For the purposes of the present invention, all of the foregoing may be collectively referred to herein as a “graft ligament”.
As noted above, various approaches are well known in the art for anchoring the two ends of the graft ligament in the femoral and tibial bone tunnels.
In one well-known procedure, which may be applied to femoral fixation, tibial fixation, or both, the end of the graft ligament is placed in the bone tunnel, and then the graft ligament is fixed in place using a headless orthopedic screw, generally known in the art as an “interference” screw. More particularly, with this approach, the end of the graft ligament is placed in the bone tunnel and then the interference screw is advanced into the bone tunnel so that the interference screw extends parallel to the bone tunnel and simultaneously engages both the graft ligament and the side wall of the bone tunnel. In this arrangement, the interference screw essentially drives the graft ligament laterally, into engagement with the opposing side wall of the bone tunnel, whereby to secure the graft ligament to the host bone with a so-called “interference fit”. Thereafter, over time (e.g., several months), the graft ligament and the host bone grow together at their points of contact so as to provide a strong, natural joinder between the ligament and the bone.
Interference screws have proven to be an effective means for securing a graft ligament in a bone tunnel. However, the interference screw itself generally takes up a substantial amount of space within the bone tunnel, which can limit the surface area contact established between the graft ligament and the side wall of the bone tunnel. This in turn limits the region of bone-to-ligament in-growth, and hence can affect the strength of the joinder. By way of example but not limitation, it has been estimated that the typical interference screw obstructs about 50% of the potential bone-to-ligament integration region.
For this reason, substantial efforts have been made to provide interference screws fabricated from absorbable materials, so that the interference screw can eventually disappear over time and bone-to-ligament in-growth can take place about the entire perimeter of the bone tunnel. To this end, various absorbable interference screws have been developed which are made from biocompatible, bioabsorbable polymers, e.g., polylactic acid (PLA), polyglycolic acid (PGA), etc. These polymers generally provide the substantial mechanical strength needed to advance the interference screw into position, and to thereafter hold the graft ligament in position while bone-to-ligament in-growth occurs, without remaining in position on a permanent basis.
In general, interference screws made from such biocompatible, bioabsorbable polymers have proven clinically successful. However, these absorbable interference screws still suffer from several disadvantages. First, clinical evidence suggests that the quality of the bone-to-ligament in-growth is somewhat different than natural bone-to-ligament in-growth, in the sense that the aforementioned bioabsorbable polymers tend to be replaced by a fibrous mass rather than a well-ordered tissue matrix. Second, clinical evidence suggests that absorption generally takes a substantial period of time, e.g., on the order of three years or so. Thus, during this absorption time, the bone-to-ligament in-growth is still significantly limited by the presence of the interference screw. Third, clinical evidence suggests that, for many patients, absorption is never complete, leaving a substantial foreign mass remaining within the body. This problem is exacerbated somewhat by the fact that absorbable interference screws generally tend to be fairly large in order to provide them with adequate strength, e.g., it is common for an interference screw to have a diameter (i.e., an outer diameter) of 8-12 mm and a length of 20-25 mm.
Thus, there is a need for a new and improved interference fixation system which (i) has the strength needed to hold the graft ligament in position while bone-to-ligament in-growth occurs, and (ii) promotes superior bone-to-ligament in-growth.